The present invention relates to an injection type noncatalyst denitrogen oxide (deNO.sub.x) process control system, and in particular to control the amount of denitrogen oxide (deNO.sub.x) automatically according to the flue gas conditions of a combustion device.
Environmental preservation has become increasingly important. Nitrogen oxide has been discovered to be the major cause of acid rain. In fact, almost all nitrogen oxides come from burning fossil fuels. As a result, stringent regulations to reduce the allowable emissions of nitrogen oxides are being promulgated in many industrial areas of the world.
The combustion industry is faced with the necessity of having to reduce emissions of nitrogen oxides from its existing units. However, conventional combustion technologies can't meet standards for low NO.sub.x emissions set by such stringent regulations. In order to meet such standards, methods for reducing nitrogen oxides in furnaces have been developed. These methods can be divided into two groups, i.e., the pre-treatment method and the post-treatment method. The pre-treatment method reduces the nitrogen oxides in the flue gas by use of the direct combustion method, i.e., by using the combustion technology of low nitrogen oxide. The post-treatment method reduces the nitrogen oxide to nitrogen by additional reducing agent (such as ammonia, urea) to the already generated flue gas. Examples of the post-treatment method include Selective Catalyst Reduction (SCR) and the Selective Non-Catalyst Reduction (SNCR). The present invention relates to the control system for the SNCR method of post-treatment.
The SNCR method was invented by the Exxon Research and Engineering Co. in 1973. The SNCR method involves injecting ammonia (NH.sub.3) into the high temperature flue gas within a temperature range of 870.degree. C. to 1200.degree. C. The NO.sub.x can be reduced to N.sub.2 and H.sub.2 O by selective reaction of NO.sub.x and NH.sub.3 during high temperature. The process of reducing NO.sub.x to N.sub.2 and H.sub.2 O requires lower capital investment than the SCR method, whereas the SCR method involves reaction by use of a catalyst at a temperature between 250.degree. C. and 400.degree. C.
The performance of the SNCR method depends on the ratio of NH.sub.3 to NO and the temperature during the reaction. The SNCR method consumes more reducing agent and is proved to be more serious ammonia slip than the SCR method, because the SNCR method has a higher ratio of NH.sub.3 to NO than the SCR method.
Besides NH.sub.3, the SNCR method can also use other reducing agents, such as urea, CH.sub.3 NH.sub.2 and (CHNO).sub.3. The SNCR method can be applied to gas and oil-fired steam boilers, utility boilers, municipal incinerators, oil field steam generators, glass melting furnaces and flue-coke furnaces.
Nowadays, almost all applications of SNCR method involve installing the injection grid within a flue gas at a proper flue gas temperature to inject reducing agent, such as NH.sub.3. The reaction occurs after mixing the reducing agent and the flue gas. Location of the injection grid is extremely important so as to achieve optimum performance of denitrogen oxide (deNOx). Unfortunately, the optimum location of adding the SNCR installation in an original system is always limited by its equipment space. In addition, the system may sometimes encounter insufficient reaction temperature during low load operation. Furthermore, rust, corrosive or heavy fouling of the injection grid can easily happen if it directly reacts with the flue gas.
The development of a new SNCR technique employs both an injection grid and a wall injector at the same time. FIG. 1 illustrates a schematic diagram of the prior art for utilizing both an injection grid and a wall injector in an oil-fired steam boiler 1, wherein 11 indicates an injection grid, 12 indicates a wall jet, 13 indicates a primary superheater, 14 indicates a secondary superheater, 15 indicates a reheat superheater, 16 indicates a burner, 17 indicates a flue gas outlet, 18 indicates an air inlet, 19 indicates flue gas flow. The new SNCR technique offers the advantages of high performance, better load following without hydrogen, grid-less injectors and lower capital investment.
However, the above mentioned new SNCR technique still has some disadvantages, i.e., the discharge amount of the reducing agent cannot auto-adjust itself in response to the reaction temperature and the amount of flue gas. The control technique for the SNCR method is substantially more important than that for the SCR method, since the denitrogen oxide (deNO.sub.x) rate and the discharge amount of NH.sub.3 are affected by the mixing of NH.sub.3 and the flue gas, the retention time of NH.sub.3, and the reaction temperature. Therefore, the highest rate of denitrogen oxide (deNO.sub.x) and the less discharge amount of NH.sub.3 can be achieved by controlling the optimum operation conditions.